In processing video signals, it is known to be particularly advantageous to separate the luminance and chrominance components from composite video signal with comb filters. The reason being that the separated luminance component is of full bandwidth, e.g., 4.2 MHz for NTSC signal, and cross components are substantially eliminated from both of the separated luminance and chrominance components. Typical intraframe comb filters for NTSC video signals include circuitry for combining signals which are displaced in time by an odd integral number of horizontal line periods. Because the phase of the chrominance subcarrier changes by exactly 180.degree. from line to line, additively combining NTSC video signals displaced by one line interval produces a signal in which the luminance components from the two lines constructively combine and the chrominance component is cancelled. Conversely, if the signals are subtractively combined the luminance component is cancelled while the chrominance components from the two lines constructively combine. The vertical resolution of the luminance component is compromised but is acceptible.
PAL signals on the otherhand have a chrominance subcarrier which exhibits a 180.degree. phase change every two horizontal line periods. Therefore, typical intraframe PAL comb filters combine video signals which are displaced in time by two horizontal intervals. The PAL comb filter function is basically the same as for the NTSC comb filter however, since the signals that are combined are spatially displaced by two lines, the vertical resolution of the PAL luminance component is significantly impaired, and tends towards not being acceptable.
Yoshimitsu Nakajima et al in an article entitled "Improvement of Picture Quality For NTSC And PAL Systems By Digital Signal Processing," IEEE Transactions on Consumer Electronics, Vol. CE-31, No. 4, November, 1985, pp. 642-654 describe adaptive comb filters which combine samples from three successive lines of video signal. In this system, the NTSC embodiment combines samples which are vertically aligned, similar to the aforedescribed typical NTSC and PAL comb filters. The PAL embodiment however combines diagonally aligned samples from adjacent lines. This tends to preserve the vertical resolution of the luminance component, reduces memory requirements for delaying the video signals but tends to have an adverse affect on images containing vertical lines.
Published West German Application No. DE 3007520 A1 shows a comb filter apparatus for PAL signals including an intraframe comb filter. The intraframe comb filter includes a one line delay element for delaying alternate samples of the video signal from successive lines of signal. The delayed samples are coupled to one input of a combining circuit and intervening samples from the current line are coupled to a second input of the combining circuit. Since the combined signals correspond to alternate samples on alternate lines, the intraframe filter function corresponds to a diagonal rather than a vertical comb filter. Further since the apparatus operates on alternate samples, there is a loss in horizontal resolution.
Richard C. Spiero in U.S. Pat. No. 4,480,265 describes a circuit for comb filtering separated PAL chrominance signals by combining signals from adjacent video lines. In this system one of the signals to be combined is first applied to a phase inverter which inverts the phase of the (R-Y) component of the chrominance signal before combining. The line of chrominance which undergoes phase inversion has its components phase aligned with the chrominance components of adjacent lines. The phase inverted line and an adjacent line is additively combined to develope a noise reduced chrominance signal. This system is limited to comb filtering chrominance signal and cannot be utilized to separate the luminance and chrominance components from composite PAL video signals.
It is an object of the present invention to provide a comb filter for processing at least PAL video signals, without any of the aforementioned shortcomings.